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How to evaluate the integral $$\int_0^{\pi/2}x^2(\sin x+\cos x)^3(\sin x\cos x)^{1/2} \, dx \text{ ?}$$ I tried to subsititution $x=\frac{\pi}{2}-t$, but it doesn't work. can someone help me, any hint or trick are appreciated.

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    $\begingroup$ where does the question come from? $\endgroup$
    – pisco
    Jan 9, 2018 at 7:21
  • $\begingroup$ @pisco125 From a math bbs,... $\endgroup$
    – JamesJ
    Jan 9, 2018 at 7:25
  • $\begingroup$ Did you try $$\int_{0}^{a} f(x)dx = \int_{0}^{a} f(a-x)dx$$ $\endgroup$ Jan 9, 2018 at 7:44
  • $\begingroup$ @SatishRamanathan I tried to subsititution x=π/2−t,but it doesn't work. $\endgroup$
    – JamesJ
    Jan 9, 2018 at 8:21

1 Answer 1

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$$\color{blue}{\int_0^{\pi/2} {{x^2}{{(\sin x + \cos x)}^3}\sqrt {\sin x\cos x} dx} = \frac{\pi}{384\sqrt{2}}\left(35 \pi ^2-84 \ln ^22+132 \ln 2-150 \right)}$$


Note that $$\int_0^{\pi/2} {{x^2}{{(\sin x + \cos x)}^3}\sqrt {\sin x\cos x} dx} = \int_0^{\pi/2} {{x^2}{{(\sin x + \cos x)}^3}\sin x\sqrt {\cot x} dx}$$ and the identity $$(\sin x + \cos x)^3 \sin x = \frac{1}{4} (4 \sin (2 x)-\sin (4 x)-2 \cos (2 x)-\cos (4 x)+3)$$ Thus it suffices to find $$I_1 = \int_0^{\pi/2} {{x^2}\sqrt {\cot x} dx} \qquad I_2 = \int_0^{\pi/2} {{x^2}\sqrt {\cot x} \cos (2x)dx} \qquad I_3 = \int_0^{\pi/2} {{x^2}\sqrt {\cot x} \cos (4x)dx}$$ $$I_4 = \int_0^{\pi/2} {{x^2}\sqrt {\cot x} \sin (2x)dx}\qquad I_5 = \int_0^{\pi/2} {{x^2}\sqrt {\cot x} \sin (4x)dx}$$


Whenever the integral converges, $$\tag{1}\int_0^\pi \frac{e^{i ax}}{\sin^b x} dx = \frac{{{2^b}\Gamma (1 - b)\pi }}{{\Gamma (1 + \frac{{a - b}}{2})\Gamma (1 - \frac{{a + b}}{2})}}{e^{ia\pi /2}}$$ this is a direct consequence of contour integration

Now, note that

$$\begin{aligned}\int_0^{\frac{\pi }{2}} {{x^2}\sqrt {\cot x} \cos rxdx} &= \sqrt 2 \int_0^{\frac{\pi }{2}} {{x^2}\frac{{\cos x}}{{\sqrt {\sin 2x} }}\cos rxdx} \\&= \frac{{\sqrt 2 }}{8}\int_0^\pi {{x^2}\frac{{\cos \frac{x}{2}}}{{\sqrt {\sin x} }}\cos \frac{{rx}}{2}dx} \\&= \frac{{\sqrt 2 }}{{16}}\int_0^\pi {\frac{{{x^2}}}{{\sqrt {\sin x} }}\left[ {\cos \frac{{(1 + r)x}}{2} + \cos \frac{{(1 - r)x}}{2}} \right]dx}\end{aligned}$$ the RHS can be evaluated by taking $b=1/2$ then differentiates $(1)$ with respect to $a$ twice.

Substituting suitable values of $r$ gives $$I_1 = \frac{{\sqrt 2 \pi }}{{96}}(5{\pi ^2} - 12\pi \ln 2 - 12{\ln ^2}2)$$ $$I_2 = \frac{\pi}{96\sqrt{2}} \left(5 \pi ^2-12 \pi +24-12 \ln ^22-12 \pi \ln 2-24 \ln 2 \right)$$ $$I_3 = \frac{\pi}{192\sqrt{2}} \left(5 \pi ^2-18 \pi +54-12 \ln ^22-12 \pi \ln 2-36 \ln 2 \right)$$

Similar trick works for $I_4, I_5$, with respective values: $$I_4 = \frac{\pi}{96\sqrt{2}} \left(5 \pi ^2-12 \pi -24-12 \ln ^22+12 \pi \ln 2+24 \ln 2 \right)$$ $$I_5 = \frac{\pi}{196\sqrt{2}} \left(5 \pi ^2-30 \pi -42-12 \ln ^22+12 \pi \ln 2+60 \ln 2 \right)$$

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  • $\begingroup$ I must say I'm very amused by this solution. I imagine there is some "foot work" not shown here to really calcuate the different integrals. In any case, well done! $\endgroup$
    – mickep
    Jan 9, 2018 at 12:34
  • $\begingroup$ Thank you very much for your answer, a wonderful answer! $\endgroup$
    – JamesJ
    Jan 9, 2018 at 15:21
  • $\begingroup$ @mickep I completely revamped my answer, although some calculations are still involved. $\endgroup$
    – pisco
    Jan 9, 2018 at 17:13
  • $\begingroup$ @pisco125 I would upvote again, if I could. Cheers! $\endgroup$
    – mickep
    Jan 9, 2018 at 18:52

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